# Negative pressure

1. Sep 14, 2010

### the4thamigo_uk

Wikipedia suggests it is possible for some gases to exert negative pressure?

'Most often, gases and liquids are not capable of negative absolute pressure, or even zero pressure...'

Is this an error or can some gases exert negative pressure?

2. Sep 14, 2010

### stewartcs

There is no such thing as negative absolute pressure. Maybe it was referring to negative gauge pressure instead. Wikipedia isn't a reputable source by the way.

CS

3. Sep 14, 2010

### the4thamigo_uk

Searching for absolute pressure comes up with the following.

* When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the transpiration pull of plants, and is used to suction water even higher than the ten metres that it rises in a pure vacuum.
* The Casimir effect can create a small attractive force due to interactions with vacuum energy; this force is sometimes termed 'vacuum pressure' (not to be confused with the negative gauge pressure of a vacuum).

So, assuming this scenario, i.e. a gas in a perfectly elastic container with particles that are attracted to each other, could it be true to say that they can exert negative pressure? At all moments we have particles that are in motion and they will exert forces on the container walls. Even when they coalesce they still will be in motion and will exert forces on the walls? Can anyone suggest what is the basis for these Wikipedia comments? Could there be any chance of some truth in this?

4. Sep 14, 2010

### cesiumfrog

You're mistaken. There is such a thing as negative absolute pressure. In a solid, you normally call it tension. In some trees, it is the mechanism thought to be responsible for lifting water higher than the 10m limit.

I don't see why it should be impossible in principle, but I don't know of any gases exhibiting it.

5. Sep 14, 2010

### Andy Resnick

I'm not sure about gases, but liquids can support tension (which IIRC is negative absolute pressure).

http://web.pdx.edu/~d4eb/tensile/index.htm
http://www.nature.com/nature/journal/v278/n5700/abs/278148a0.html

I don't think gases can support tension, because there are no intermolecular bonds.

6. Sep 14, 2010

### stewartcs

No I'm not, you are mistaken.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html

In a solid, "pressure" is referred to as stress. If it is negative, it is called compression. If it is positive it is called tension. With respect to fluids, by definition they cannot be negative since a fluid cannot support a shear stress. In other words, it cannot be "pulled apart" or placed in tension.

They only exception I know of is with Surface Tension but that is a special phenomenon that only occurs at the surface of a liquid.

CS

7. Sep 14, 2010

### cesiumfrog

Heh. Look, for starters, compression relates to positive pressure. And of course I'll concede that an ideal gas cannot exert negative pressure! Now have you thought about how surface tension works at the molecular level?

Last edited by a moderator: May 4, 2017
8. Sep 15, 2010

### DrDu

If you consider e.g. the van der Waals isothermal curves. There are regions where the curve really drops below 0 of p. These regions which assume a homogeneous system are unstable against a separation into a liquid and gaseous phase. The question arises whether they are at least meta-stable. However, they correspond to a super-heated liquid rather than a gaseous phase.

9. Sep 15, 2010

### Andy Resnick

I'm going to change my answer- tension is not a negative absolute pressure- that would correspond to negative absolute energy density and is not physical.

But fluids can indeed support tension- it's related to the onset of cavitation.

10. Sep 15, 2010

### stewartcs

Sorry, I believe I misread what you wrote with respect to tension being equated to negative pressure. That part is true. However, it is normally accepted in Fluid Mechanics that negative absolute pressures do not exist since that implies fluids could sustain a tensile force.

Axial stress (i.e. mechanical pressure) in Mechanics is the equivalent to fluid pressure. There are two types of axial stress; tensile and compressive. The standard convention in Statics or Strength of Materials is for the tension to be positive and compression negative. I misread what you wrote and thought you had implied the opposite. Which you did not.

Source: Fluid Mechanics, 9th Edition - Streeter, Wylie, Bedford. 1998, McGraw-Hill Companies.

This excerpt from one of my Fluid Mechanics books shows the common ruling on tensile stress in liquids (we all seem to agree that fluids in a gas phase cannot be negative).

Honestly, I'm not sure what conditions constitutes "abnormal" (other than Surface Tension which I've already pointed out) so I'll concede that there may, emphasis on may, be some situation that would allow one to say a fluid can sustain a tensile stress other than through Surface Tension. I'd appreciate any reputable and properly vetted sources for further reading if anyone has them.

CS

11. Sep 15, 2010

### kcdodd

If water can not sustain tension, then how can water drops form on the bottom of a/c vents on the ceiling?

12. Sep 15, 2010

### cmos

Not to add more confusions to the mix, but in engineering practice, we will often say something is "under negative pressure" if it's pressure is lower than atmosphere. For example, an arsenic-containing apparatus might be kept "under negative pressure" so that, in the event of an interlock failure, arsenic is not seeped out into the air. Obviously, the "negative" here refers more to the pressure gradient than the actual pressure.

13. Sep 15, 2010

### Studiot

This has already been discussed. It is referred to as gauge pressure.

I wonder if the original poster's reference was referring to the "hydrostatic pressure" that appears in the decomposition of stress at a point into a hydrostatic stress matrix and a deviator stress matrix.

This "hydrostatic stress" can be positive or negative and is sometimes called hydrostatic pressure.

14. Sep 15, 2010

### stewartcs

As previously discussed, a phenomenon called Surface Tension is responsible for the water droplets.

CS

15. Sep 15, 2010

### kcdodd

And what is the surface of water made out of?

16. Sep 15, 2010

### Andy Resnick

What's wrong with the Nature article I posted?

17. Sep 15, 2010

### kcdodd

Yes, Andy, clearly intermolecular bonds can support tension, otherwise surface tension would be impossible to begin with. These bonds are also present throughout the bulk liquid, which means there could be tension in bulk liquid as well, not just at the surface. Any surface which can make a similar bond to the water can clearly "pull" on the water in such a case. Otherwise water droplets could not stick to the ceiling.

Also, negative energy is valid. Just not negative kinetic energy. You can set zero anywhere. Say you have two separate water droplets. If they join into one droplet the energy in the bonds will drop. Otherwise they would not join. If you set the binding energy to zero before, then it becomes negative. I think the best place to put zero is when all water molecules are separate, which would mean any droplet has negative binding energy.

Last edited: Sep 15, 2010
18. Sep 15, 2010

### stewartcs

How about we start with why, other than surface tension, you think a liquid supports a tensile stress?

CS

19. Sep 15, 2010

### stewartcs

Clearly, that only applies to the surface of a liquid. Otherwise they wouldn't call it Surface Tension, it would be called Fluid Tension or something similar. Those special bonds are not present other than at the surface, which means that a liquid will generally not support a tensile stress as stated clearly in most all Fluid Mechanics books.

CS

20. Sep 15, 2010

### Studiot

Not quite accurate here.
The bonds are indeed present throughout the liquid. That's the whole point.

There are none outside the liquid so there is a net attraction into the liquid on the particles at the surface, unbalanced by any force from the outside.

this doesn't mean I'm suggesting a fluid can support internal tension, just that your proposed mechanism for surface tension need adjusting.